1. Field of the Invention
This invention relates to a focusing controller for photographic apparatuses and, more specifically, to a focusing controller for photographic apparatuses which effects focusing by shifting a lens by a lens shifting amount converted from a distance signal calculated by and emitted from a distance meter.
2. Related Background Art
In controllers of this type, the conversion to a lens shifting distance of the distance signal, calculated by and emitted from the distance meter, has conventionally been controlled on the basis of the following principle:
First, the relationship between film-to-object distance and lens shifting distance for focusing will be explained with reference to FIG. 1, which is a diagram illustrating the relationship in terms of distance between an object, a film, and lens focuses.
In FIG. 1, the reference symbol x' indicates the distance between the film and the lens focus on the film side. The reference symbol f' indicates the distance between the lens and the lens focus on the film side. The reference symbol f indicates the distance between the lens and the lens focus on the object side. The reference symbol x indicates the distance between the lens focus on the object side and the object, and the reference symbol R indicates the distance between the film and the object.
From the image formation formula, the relationship between the above distances can be expressed as: EQU f=f' EQU x.multidot.x'=f.multidot.f'
And, the distance between the object and the film is expressed as: EQU R=f+f'+x+x'
Thus, by shifting the lens towards or away from the film surface, the above distance x' is changed, thereby effecting focusing (This operation will be hereinafter referred to simply as "shifting").
Next, a distance signal which is emitted from an active-type distance meter will be explained with reference to FIG. 2, which is a diagram illustrating the principle on the basis of which a light reception position detecting element performs detection with respect to film-to-object distance.
In FIG. 2, the reference symbol P1 indicates the position of a light projecting element.
The light projecting element projects modulated light onto an object. The reference symbol P2 indicates the position of the object, on the surface of which occurs diffused reflection of the modulated light from the light projecting element. The reference symbol P4 indicates a reference position for distance meter values, where a light projection lens is arranged. L is the distance between this reference position and the object. The light projection lens serves to stop down the modulated light from the light projecting element. The reference symbol P6 indicates the film position. Assuming that the distance between P4 and P6 is 6, the above-mentioned value R can be expressed as: EQU R=L+.delta.
The reference symbol P5 indicates a lens which causes the projection light, reflected by the object, to strike upon a photoreceptor.
The reference symbol P3 indicates a light-reception-position detecting element, which detects changes in a value b due to changes in the above L to calculate the value of 1/L and, further, emit the value of 1/R. Here, b is the value of a distance signal indicating the distance between the light-reception position and the reference position.
The relationship between b and L can be expressed as: EQU L: x0=y0:b
Accordingly; EQU b=x0-y0/L
Thus, the value of b is in proportion to 1/L.
Further, as an electrical signal, there is emitted a distance, signal a, which is an electrical signal corresponding to the position indicated by the value b. If the light-reception-position detecting element consists of a PSD (a semiconductor device), the distance signal a can be calculated from two signals I1 and I2 emitted from the PSD, by using the following equation: ##EQU1##
Next, the relationship between film-to-object distance and lens shifting distance will be described with reference to FIG. 3, which is a graph illustrating a conventional method of controlling lens shifting distance. In the example shown in this graph, the relationship between lens shifting distance x' and a value 1/R is such that when the focal length is 35 mm, which is a focal length value generally adopted in cameras, 1/R can be expressed as a value which varies at equal intervals. Here, R indicates film-to-object distance and 1/R is the reciprocal thereof. The horizontal axis of the graph represents lens shifting distance and the vertical axis thereof represents the reciprocal of film-to-object distance, 1/R. Shown in the parentheses beside the vertical axis are values of R given in the unit of 0.1 m. As stated above, the distance signal a and the reciprocal of film-to-object distance, 1/R, are in one-to-one correspondence with each other and can be represented in the same axis.
As stated above, in the conventional control method, the reciprocal of film-to-object distance, 1/R, is a value which varies at equal intervals, with the result that the relationship between lens shifting distance and the reciprocal of film-to-object distance, 1/R, is such as to deviate from a linear expression, with the lens shifting distance x' being controlled as a value varying at unequal intervals.
Thus, in the prior art, focusing control has been effected by controlling lens shifting amount at unequal intervals, as stated above. Accordingly, the conventional focusing controller involves a considerable burden in terms of lens shifting control and the mechanism thereof, making the control operation and device structure rather complicated. Therefore, the conversion of distance signal to lens shifting distance and the correction and adjustment of the same have been by no means easy to perform, which has been one of the causes of high production costs. This is due to the fact that the correction and adjustment of lens shifting distance have been effected by adjusting different components of mechanism sections such as the optical system and the cam barrel, which require a well-balanced mutual setting.